The well known pasteurization process to kill bacteria in milk has been used for many decades. Unfortunately, the higher temperatures needed in the pasteurization process adversely affect the flavor of the milk. Further, even with the use of such high temperatures, the pasteurization process does not eliminate all undesirable bacteria, leading to the short storage stability of most milk products.
Bacillus cereus are often the predominant bacteria in conventionally processed milk of relatively advanced age, because they can survive the pasteurization process and they thrive at cold temperatures, promoting the spoilage of the milk. A general need exists for a method for reducing the content of bacteria in milk, both whole and skim milk, to enhance the storage stability of the product and to improve its flavor by elimination of the pasteurization process.
Various methods for producing milk with a lowered bacterial count through the use of filtration have been known in the art, but none have found wide acceptance. The prior art methods generally provide either poor flow rates, rendering the method uneconomical on a large scale, or adversely affect the quality of the milk, making the product unacceptable to consumers.
It is clear, then, that the pores of bacteria filters used in the art, which filters are effective to sterilize milk, also will remove not only the bacteria, but also the fat globules, and at least some of the proteins. Such a filter quickly becomes blocked by trapped material; hence, the flow rate through the filter rapidly declines and the filter must be frequently cleaned or replaced. The high cost of such an inefficient process is generally prohibitive. Further, because the filter holds fat globules and proteins, the quality of the milk is also adversely affected.
From the foregoing discussion, it is apparent that there is a continuing need for an improved milk filtration processing method that can provide a sterile, or more nearly sterile product, that has an improved storage life, that and does not adversely affect milk quality.
Some attempts have heretofore been made to use cross-flow, or tangential flow, filtration devices to treat milk, such devices being known in the art.
Cross-flow filtration is substantially different from through-flow filtration, in that the liquid feed is introduced parallel to the filter surface, and filtration occurs in a direction perpendicular to the direction of the feed flow. In cross-flow filtration systems, generally, because the direction of the feed flow is tangential to the membrane surface, accumulation of the filtered solids on the filtering medium is reduced by the shearing action of the flow. Cross-flow filtration thus affords the possibility of a quasi-steady state operation with a nearly constant flux when the driving pressure differential is held constant. Those particles initially entering into the wall matrix ultimately become entrapped within it, because of the irregular and tortuous nature of the pore structure. As microfiltration proceeds, penetration of additional small particles into the wall matrix is inhibited by the presence of the dynamic membrane. The formation of the dynamic membrane, together with the possible clogging of the pore structure of the tube by entrapped particles, results in a decline in the filtration flux. In conventional systems, this decline is approximately exponentially related to filtration time.
Crossflow filtration of milk has been attempted, but has not been generally accepted because of the problems discussed above.
Clearly, the use of crossflow filtration, to date, has not provided an acceptable method for reducing bacterial contamination in milk.
One means to overcome some of the problems associated with classical crossflow filtration technology, known as dynamic microfiltration, has emerged. The dynamic filtration process overcomes the disadvantage in the classical crossflow technology because the liquid to be filtered is not simply guided tangentially over the membrane surface. The membrane surface or a solid body near the membrane surface is moved such that the fluid at the interface between the rotor and the stator is subjected to shearing action. The shearing action tends to "scrub" the membrane surface, keeping it relatively clear of particulate material, and preventing a filter cake from forming on the membrane surface. The particulate material that would otherwise collect on the membrane surface remains suspended, and is ultimately removed in the secondary stream, generally referred to as a concentrate stream.
It was previously discovered, as set forth fully in copending application Ser. No. 07/901,238, filed Jun. 19, 1992, that dynamic microfiltration of milk can be successfully accomplished, without the prior art problems of degradation of milk quality, premature filter plugging, and inadequate bacterial removal, through the use of dynamic microfiltration.
In accordance with the aforementioned application Ser. No. 07/901,238, milk, either whole or skim milk, is first homogenized and then subjected to filtration. By performing the homogenization step first, the particle size of the fat globules and other large, suspended components of the milk is significantly reduced, allowing for microfiltration of the milk without significant removal and entrainment of the fat and other components.
Milk is an emulsion of fat and protein particles in water. Homogenization provides for a method of reducing the emulsion particle size to allow passage through an appropriately sized microporous membrane, to retain bacteria contained therein without unwanted removal of the fat and protein content of the milk.
The milk, after homogenization, is filtered through the use of dynamic microfiltration. The invention described in application Ser. No. 07/901,238, thus provides an improved method for producing milk with a lowered bacterial content, without the need for pasteurization. That portion of the milk fraction that is retained by the microfilter (the concentrate fraction), may be recirculated as part of the feed, or may be discarded or used in other processes.
In particular, the method of said application Ser. No. 07/901,238, comprises homogenizing the milk and within about 5 minutes from the homogenization, subjecting the milk to dynamic microfiltration by passing the milk through a microfilter having an average pore size sufficient to reduce the bacterial content of the milk flowing therethrough, to yield a filtrate which has a lower bacterial content than the initial raw milk and a concentrate having a higher bacterial content than the initial raw milk. The resulting milk has a very low bacterial content, such as about 10.sup.3 bacteria per milliliter, or less, and retains more organoleptic components than that found in pasteurized milk with the same bacterial content.
Although the method of said application Ser. No. 07/901,238 provides a useful means of accomplishing the goal of milk filtration, all as discussed above and in said application, which is hereby incorporated by reference in its entirety to the full extent as if it were set forth herein, one area of possible improvement related to said method concerns the drop in filtration flow rate with time. Although the method of said application accomplishes a result not previously achievable, the filtration of milk to provide a substantially sterile product with better storage, taste, and other properties than pasteurized milk, the method does present the problem that as filtration time becomes extended, the flow rate of the milk through the membrane diminishes, resulting in decreased efficiencies in the method, with time. If the process is continued, ultimately flow through the membrane will essentially cease. Of course, at some time significantly before ultimate cessation of flow, the method becomes uneconomical to operate commercially. It may not be possible to simply replace the membranes used in the dynamic filtration, due to the cost of the membranes, as well as to the cost associated with frequent changing of such membranes.
The flow of milk through, for example, a microfilter employing a 0.45 .mu.m membrane, diminishes, as a general matter, at a rate such that after about 9 hours of milk filtration, the flow rate of milk through the membrane has decreased to about one-half of the initial milk flow rate, at a constant pressure.
A need therefore exists for a means to modify the method of the 07/901,238 application to overcome the problem of reduced flow rates with the passage of time, to thus render the method more economically acceptable.